Process for fabricating polymeric articles

ABSTRACT

A process for the production of a polymeric article directed to (a) forming a ply having successive layers, namely, (i) a first layer made up of strands of an oriented polymer material; (ii) a second layer of a polymeric material; (iii) a third layer made up of strands of an oriented polymeric material, wherein the second layer has a lower peak melting temperature that of the first and third layers; (b) subjecting the ply to conditions of time, temperature, and pressure sufficient to melt a proportion of the firsts layer to melt the second layer entirely, and to melt a proportion of the third layer, and to compact the ply; and (c) cooling the compacted ply. The resultant articles have good mechanical properties yet may be made at a lower compaction temperature than articles not employing the second layer, leading to a more controllable manufacturing process.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 15/195,277,filed Jun. 28, 2016, now issued as U.S. Pat. No. 9,873,239 which is acontinuation of Ser. No. 14/502,377, filed Sep. 30, 2014, (now U.S. Pat.No. 9,403,341, issued Aug. 2, 2016) which is a continuation of Ser. No.13/561,330, filed Jul. 30, 2012, (now U.S. Pat. No. 8,871,333, issuedOct. 28, 2014) which is a continuation of Ser. No. 13/276,118, filedOct. 18, 2011, (now U.S. Pat. No. 8,268,439, issued Sep. 18, 2012),which is a divisional of application Ser. No. 10/556,402, filed on Jan.23, 2006, (now U.S. Pat. No. 8,052,913 B2, issued Nov. 8, 2011), whichis the national stage entry of PCT/GB04/02184, filed May 21, 2004,claiming priority to GB 0311819.7, filed May 22, 2003 and EP 03253211.1,filed May 25, 2003, whose disclosures are incorporated herein byreference.

The present invention relates to polymeric articles made from orientedpolymeric strands, and in particular to an improved process for makingsuch articles.

BACKGROUND OF THE INVENTION

In recent years, developments have been made in processes for compactingpolymeric strands in order to make sheets of high stiffness andstrength. An example is disclosed in GB 2253420A, in which an assemblyof strands of an oriented polymer is hot compacted in a two-step processto form a sheet having good mechanical properties. The process involvesan initial step in which the strands are brought to and held at thecompaction temperature whilst subject to a pressure sufficient tomaintain the strands in contact, and thereafter compacted at a highpressure (40-50 MPa) for a few seconds (the compaction pressure). Inthis process a proportion of the surfaces of the strands melts andsubsequently recrystallises on cooling. This recrystallised phase bindsthe strands together, resulting in good mechanical properties of thefinal sheet. It is mentioned in GB 2253420A that the process can beapplied to many types of oriented polymer including polyester and PEEK(polyether ether ketone) but that preferred polymers are orientedpolyolefins.

One drawback of the process described in GB 2253420A is that thetemperature span across which melting occurs is very narrow. Accordinglyit is difficult to achieve the desired degree of partial melting of theouter regions of the strands. Inadequate melting of the strands resultsin poor mechanical properties. Excessive melting of the strands resultsin loss of orientation, and diminished mechanical properties. Preciseprocess control is needed if the article is not to be “under-melted” or“over-melted”.

In WO 98/15397 a related process is disclosed in which an assembly ofmelt-formed polyolefin strands is maintained in intimate contact atelevated temperature sufficient to melt a proportion of the strands,whilst being subjected to a compaction pressure of no greater than 10MPa. If wished the strands may have been subjected to a priorcrosslinking process, preferably an irradiation crosslinking processcomprising irradiating the strands with an ionising radiation in aninert environment containing alkyne or diene compounds, and thencarrying out an annealing step comprising annealing the irradiatedpolymer at an elevated temperature, in an inert environment containingalkyne or diene compounds. It is said that the prior crosslinking canmake the compaction temperature less critical, and improve mechanicalproperties, in particular the failure strength at elevated temperature.

There is published work on the use of articles in which a polyethylenefilm is sandwiched between polyethylene fibre layers, and the plysubjected to hot compaction.

Marais et al., in Composites Science and Technology, 45, 1992, pp.247-255, disclose a process in which compaction takes place at atemperature above the melting point of the film but below the meltingpoint of the fibre layers. The resulting articles have modest mechanicalproperties.

Ogawa et al., in Journal of Applied Polymer Science, 68, 1998, pp.1431-1439 describe articles made up of layers of ultra-high molecularweight polyethylene fibres (mp 145-152° C.) and low density polyethylenefilms (mp 118° C.). The moulding temperature is said to be between themelting points of the fibre and the interlayer (matrix). The volumefraction of the fibres is stated to be 0.69 or 0.74. However thearticles are said to have surprisingly poor properties, possibly becauseof weak adhesion between fibres and matrix (melted film). Anotherarticle was made with polyethylene fibres alone, and the processconditions induced partial melting, with poorer properties.

SUMMARY OF THE INVENTION

There is a need for a simple, practical means which can reduce thecriticality of the compaction temperature, in a hot compaction process.There is in addition a continuing need for improvement in mechanicalproperties in the resulting articles. It is an object of the presentinvention to achieve embodiments in which one or both of these needs aremet, at least in part, in a practicable manner.

Accordingly in a first aspect of the present invention there is provideda process for the production of a polymeric article, the processcomprising the steps of:

(a) forming a ply having successive layers, namely

(i) a first layer made up of strands of an oriented polymeric material;

(ii) a second layer of a polymeric material;

(iii) a third layer made up of strands of an oriented polymericmaterial, wherein the second layer has a lower peak melting temperaturethan that of the first and third layers;

(b) subjecting the ply to conditions of time, temperature and pressuresufficient to melt a proportion of the first layer, to melt the secondlayer entirely, and to melt a proportion of the third layer; and tocompact the ply; and

(c) cooling the compacted ply.

“Cooling” in the first and second aspects can include permitting thecompacted ply to cool naturally; forced draught cooling; plunge cooling;any other type of accelerated cooling; and retarded cooling.

The term “strands” is used herein to denote all oriented elongateelements of polymeric material useful in this invention. They may be inthe form of fibres or filaments. They may be in the form of bands,ribbons or tapes, formed for example by slitting melt formed films, orby extrusion. Whatever their form the strands may be laid in a non-wovenweb for the process of the invention. Alternatively they may be formedinto yarns comprising multiple filaments or fibres, or used in the formof a monofilament yarn. The strands are usually formed into a fabric byweaving or knitting. Optionally the strands may have been subjected to acrosslinking process, as described in WO 98/15397. Woven fabrics arepreferably made up of tapes, fibre yarns or filament yarns, or they maycomprise a mixture of fibre or filament yarns and tapes. Most preferredfor use in the said first and third layers are fabrics which are wovenfrom flat tapes, as this geometry is believed to give the besttranslation of the oriented phase properties into the properties of thefinal compacted sheet.

The strands can be made by any suitable process, for example solution orgel or melt forming, preferably by melt forming.

Preferably at least 1% of each of the first layer melts, preferably atleast 3%, more preferably at least 5%. Especially preferred areembodiments in which at least 10% of the first layer melts (vol/vol offirst layer).

Preferably not more than 30% of the first layer melts, more preferablynot more than 25%. Highly preferred are embodiments in which not morethan 20% of the first layer melts, and especially not more than 15%(vol/vol of the first layer).

Preferably at least 1% of each of the third layer melts, preferably atleast 3%, more preferably at least 5%. Especially preferred areembodiments in which at least 10% of the third layer melts (vol/vol ofthird layer).

Preferably not more than 30% of the third layer melts, more preferablynot more than 25%. Highly preferred are embodiments in which not morethan 20% of the third layer melts, and especially not more than 15%(vol/vol of the third layer).

Preferably at least 1% of the ply melts, preferably at least 3%, morepreferably at least 5%, and most preferably at least 10% (vol/vol oftotal ply).

Preferably not more than 35% of the ply melts, preferably not more than25%, more preferably not more than 20%, and most preferably not morethan 15% (vol/vol of total ply).

Preferably the ply comprises a plurality of layers of the type definedabove as the second layer, for example from 2 to 40, preferably from 4to 30, each such layer being sandwiched between layers of the typedefined above as the first and third layers.

In certain embodiments of the invention the strands of an orientedpolymeric material of the first and third layers may comprise—preferablymay consist of—polyethylene, polypropylene, polyoxymethylene orpolyester, including as homopolymer, copolymer or terpolymer. Polymerblends and filled polymers could be employed in certain embodiments. Inespecially preferred embodiments the strands are of a homopolymericmaterial, most preferably a polypropylene or polyethylene homopolymer.

In certain embodiments of the invention the or each second layer maycomprise—preferably may consist of—polyethylene, polypropylene,polyoxymethylene or polyester, including as homopolymer, copolymer orterpolymer. Polymer blends and filled polymers could be employed incertain embodiments. In especially preferred embodiments the or eachsecond layer is of a homopolymeric material, most preferably apolypropylene or polyethylene homopolymer.

Preferably the first and third layers are of the same type of polymericmaterial (e.g., both polypropylene). Preferably the second layer is ofthe same type of polymeric material. Most preferably the second layer isof the same chemical composition and grade, except for the fact that itis preferably of lower orientation (and accordingly melts at a lowertemperature than the first and third layers).

The minimum temperature at which the fibres should be compacted ispreferably that at which the leading edge of the endotherm, measured byDifferential Scanning Calorimetry (DSC), of the constrained polymerfibres extrapolated to zero intersects the temperature axis. Preferably,the temperature at which the fibres are compacted is no greater than theconstrained peak temperature of melting at the ambient compactionpressure—i.e. the temperature at which the endotherm reaches its highestpoint.

The second layer could be formed in situ on the first or third layer,for example by delivering the polymeric material of the second layer tothe respective first or third layer in particulate form, for example byspraying.

Alternatively, and preferably, the second layer is pre-formed, and islaid onto the first or third layer. The second layer could be pre-formedfrom strands of the polymeric material. The strands could be laid into anon-woven web. They could be formed into yarns comprising multiplefilaments or fibres, or used in the form of a monofilament yarn.Strands—for example filament yarns, fibre yarns or tapes—could be formedinto a fabric by weaving or knitting. Most preferably, however, thesecond layer comprises—preferably consists of—a film. The film maytypically have a uniaxial or biaxial orientation resulting from itsformation, but such that the degree of orientation will typically bemuch less than that of the strands which make up the first and thirdlayers. The or each second layer may be made up of a plurality of films,for example 2-5, but is preferably constituted by a single film.

Preferably the or each second layer (however constituted) is ofthickness not exceeding 100 μm, more preferably not exceeding 40 μm, andmost preferably not exceeding 20 μm (with reference to its thicknesswhen under compression in the ply, at a temperature below its meltingtemperature).

Preferably the or each second layer (however constituted) is ofthickness at least 5 μm, more preferably at least 10 μm (with referenceto its thickness when under compression in the ply, but below itsmelting temperature).

Preferably the thickness of each of the first and third layers exceedsthat of the or each second layer. Preferably the thickness of each is atleast 5 times that of the or each second layer.

Preferably the thickness of each of the first and third layers exceeds50 μm, and more preferably exceeds 100 μm.

Preferably the thickness of each of the first and third layers does notexceed 1 mm, and preferably does not exceed 400 μm.

Preferably the or each second layer has a peak melting temperature atleast 5° C. lower than the peak melting temperature of the first andthird layers, more preferably at least 10° C. lower, most preferably atleast 20° C. lower.

It is preferred that the hot compaction process of the invention uses acompaction pressure not exceeding 10 MPa. It is also preferred that asingle pressure is used throughout the hot compaction process. Mostpreferred pressures are between 1 and 7 MPa, particularly between 2 and5 MPa. It is preferred that the hot compaction pressure is maintainedduring cooling.

Preferably the polymeric materials have not been subjected to acrosslinking process prior to compaction, for example of the typedescribed in WO 98/15397. It is found that the present invention givesbenefits in terms of the “temperature window” without the need forcrosslinking.

Preferably the polymeric materials have not been subjected to a priorcorona discharge treatment prior to compaction. More preferably thepolymeric materials have not been subjected to surface treatment priorto compaction.

Compaction of the polymeric materials may be carried out in anautoclave, or in a double belt press or other apparatus in which theassembly is fed through a compaction zone where it is subjected to therequired elevated temperature and pressure. Thus, the process may beoperated as a continuous or semi-continuous process. Cooling ispreferably effected whilst the compacted web is restrained againstdimensional change, for example by being held under tension, which maybe applied uniaxially or biaxially, or by being still under a compactionpressure. The restraint may assist the maintenance of good properties inthe oriented phase.

The article may be regarded as a polymer composite made up of aninterlayer or binding phase produced during the process, derived fromfull melting of the second layer and partial melting of the first andthird layers, and an oriented phase, being the unmelted major proportionof the fibres of the first and third layers.

By means of the present invention articles can be made with certainmechanical properties exceeding those which would be obtained using aconventional process which does not employ a melted second layer. Inparticular peel strength and failure strength may be significantlyimproved, with tensile modulus being maintained at a good level.

In accordance with a second aspect of the present invention there isprovided an article made by a process of the first aspect.

Articles made by the process of the present invention are suitable forforming into shape, by a process carried out subsequent to compaction(post-forming).

In accordance with a third aspect of the present invention there isprovided a process for forming a shaped article by the application ofheat and a shaping force to an article of the third aspect of thepresent invention. Suitably the article of the third aspect may be aflat sheet and the shaped article may, for example, be bent, curved,domed or otherwise non-planar.

In accordance with a fourth aspect of the present invention there isprovided an article formed into a shape by a process of the thirdaspect.

In accordance with a fifth aspect of the present invention there isprovided a ply as defined by step (a) of the first aspect, prior to thecarrying out of steps (b) and (c) of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings, in which:

FIG. 1 is a graph of tensile modulus versus temperature;

FIG. 2 is a graph of peel strength versus temperature;

FIG. 3 is a graph of tensile strength versus temperature;

FIG. 4 is a graph of performance index versus temperature;

FIG. 5 is a graph of peel strength versus temperature;

FIG. 6 is a graph of tensile modulus versus temperature;

FIG. 7 is a graph of tensile strength versus temperature;

FIG. 8 is a graph of performance index versus temperature;

FIG. 9 is a low magnification micrograph (×50) showing the sample edgeand fracture structure;

FIG. 10 is a micrograph of the sample of FIG. 9 (×30) showing the peelfracture surface for the sample made at 175° C. without a film.

FIG. 11 is a low magnification micrograph (×50) of the sample edge;

FIG. 12 is a micrograph of the sample of FIG. 11 (×30) showing surfacedamage associated with the interface where the film was located;

FIG. 13 is a low magnification micrograph (×50) showing the sample edgeand fracture surface.

FIG. 14 is a micrograph of the sample of FIG. 13 (×30) showing the peelfracture surface for the sample made at 191° C. without a film.

FIG. 15 is a low magnification micrograph (×50) of the sample edge;

FIG. 16 is a micrograph is the sample of FIG. 15 (×30) showing that asample made at 191° C. with a film develops surface damaging on peeling;

FIG. 17 is a micrograph (×30) showing a peel fracture surface from asample made at 193° C. without a film.

FIG. 18 is a micrograph of the sample of FIG. 17 (×30) showing regionswhere there has been cohesive failure at the film/tape interface;

FIG. 19 shows the results for both the flexural modulus and flexuralstrength for example set E;

FIG. 20 and FIG. 21 are views of a corresponding peel tested productthat have two and three layers of the 100GA02 polymer film,respectively;

FIGS. 22-25 show low magnification micrographs of typical fracturesurfaces from samples made without a film at 135°, 148°, 152°, and 154°C., respectively;

FIGS. 26-29 are four micrographs showing samples made with a film at135°, 148°, 152°, and 154° C., respectively;

FIG. 30 is at 135° C. with no film;

FIG. 31 is at 135° C. with film;

FIG. 32 is at 148° C. with no film;

FIG. 33 is at 148° C. with film;

FIG. 34 is at 152° C. with no film;

FIG. 35 is at 152° C. with film;

FIG. 36 is at 154° C. with no film;

FIG. 37 is at 154° C. with film.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further exemplified, with reference to thefollowing examples, set out in sets.

In these examples standard test methods were used.

Tensile modulus and tensile strength were determined following theprotocols of ASTM D638. Flexural strength was determined following theprotocols of ASTM D790.

Peel strength was determined by the protocols of the T-Peel test, ASTMD1876. Samples for testing were 10 mm wide and 100 mm long and weretested using a crosshead speed of 100 mm/min. The testing was carriedout parallel to the warp direction.

In all cases three samples were tested and the results averaged.

The percentage of material melted was determined by DifferentialScanning calorimetry (DSC) carried out at a heating rate of 10° C./min.

Example Set A

Fabric layers were woven, in a plain weave, from CERTRAN, a 250 deniermultifilament yarn of melt spun filaments of oriented homopolymericpolyethylene available from Hoechst Celanese, and characterised asfollows:

TABLE 1 Molecular weight Breaking Tensile initial Modulus (Mw) (Mn)strength (GPa) secant (GPa) 2% (GPa) 130,000 12,000 1.3 58 43

Samples, using two layers of woven cloth, were processed in a hot pressusing a two stage pressure process. An initial pressure of 0.7 MPa (100psi) was applied while the assembly reached the compaction temperature.After a 2 minute dwell time at this temperature, a higher pressure of2.8 MPa (400 psi) was applied for 1 minute upon which time the assemblywas cooled at a rate of approximately 20° C. per minute to 100° C.Samples were made under three conditions. Firstly, standard compactionat a temperature of 138° C. Secondly, a layer of the LDPE film was laidbetween the two layers of woven cloth and then processed at 126° C.(above the melting point of the film but below the melting point of theoriented fibres). Finally a sample was made by interleaving one layer ofthe LDPE film between the two layers of woven cloth and processing at atemperature of 136° C.

The results of these tests are shown in the table below.

TABLE 2 Compaction % fibre Peel Tensile temperature melted strengthmodulus Sample (° C.) material (N/10 mm) (GPa) Standard compaction 13826 7.2 9.2 technique (comparison) Woven PE cloth + 126 0 6.8 3.1interleaved LDPE film (comparison) Woven PE cloth + 136 14 11.2 8.1interleaved LDPE film

For the standard compaction technique without the film, a compactiontemperature of 138° C. was found to be optimum for giving a good modulusand reasonable level of interlayer bonding (peel strength). This optimumtemperature was very close to the point where major crystalline incitingoccurred, at 139° C. Using an interleaved film, but processing at 126°C., just enough to completely melt the interlayer film, but not thesurfaces of the fibres, good interlayer bonding was developed butmodulus was poor due, presumably, to poor interfibre bonding as it willbe difficult for the molten material to penetrate the fibre bundles.Finally, the sample made with the interlayer film but processed at 136°C., where selective inciting of the oriented fibres occurred, shows thehighest peel strength and a good modulus. In addition, those propertieswere obtained at a temperature 2° C. below the temperature required forcompaction without the film, widening the processing window as there isless risk of over melting at a temperature of 139° C.

Example Set B

In these examples partially melted monolithic articles were prepared,using TENSYLON oriented polyethylene tape produced by SyntheticIndustries, USA, having the following characteristics:

TABLE 3 Tensile strength 1.5 GPa Tensile modulus  88 GPa Denier 720

This was woven into a fabric. For the interlayer a polyethylene ofclosely similar type was obtained, FL5580 film grade from Borealis A/S,Denmark, melting point 130° C. This was extruded into a filmapproximately 10-15 μm in thickness, using a standard film extruder andfilm die.

Compaction experiments were carried out at a range of temperaturesbetween the melting point of the film (approximately 130° C.) up to andincluding the normal compaction range for this material (148°−156° C.).The woven cloth was thin (areal density 83 g/m²) and to obtain an evenpressure over the assembly during compaction rubber sheets were usedinside the normal metal plates utilised for compaction, with softaluminum foils between the rubber sheets and the ply being compacted.Dwell time was 5 minutes. Cooling was 20° C./min.

In the first series of tests, samples were compacted over thetemperature range 148 to 156° C., with and without the interleaved film.FIGS. 1, 2 and 3 show the tensile modulus, peel strength and tensilestrength of these samples.

It will be seen from FIG. 1 that when an interlayer is used, the tensilemodulus shows a monotonic decrease with temperature, as opposed to thepeak seen with normal compaction. We infer that the interlayer isproducing higher levels of bonding at low compaction temperatures makingthe properties less sensitive to the amount of melted material produced.

The peel strength of the interleaved film samples (FIG. 2) is higherthroughout the temperature range, compared to normal compaction.

The tensile strength (FIG. 3) was similar for the two samples; concernthat this property might be compromised by use of the interlayer wasallayed.

We have developed a performance index (PI) in an attempt to discern theoptimum combination of the compacted sheet properties. If we considerthe tensile modulus E, the tensile strength c and the peel strength,Peel, assuming each property is equally important, this is defined asfollows:PI=100×[(E _(T) /E _(max))+(σ_(T)/σ_(max))+(Peel_(T)/−Peel_(max))]/3where the subscript T refers to a particular compaction temperature andthe subscript max refers to the maximum value measured for all thesamples. Values of the performance index are shown below in FIG. 4. Fromthis it is seen that the interlayer samples show a less variablecombination of properties, in particular having better properties atlower compaction temperatures, than corresponding samples without aninterlayer. This confirms the view that a lower compaction temperaturecan be used when an interlayer is employed, giving processingadvantages.

Example Set C

The tests of this example employed the same materials, equipment andtechniques as Example Set B. It provides a comparison of the propertiesof compacted sheets made at three temperatures: a normal compactedsample made at the standard optimum temperature of 154° C., aninterlayer sample made at 152° C. and a comparison interlayer samplemade at 135° C., which is enough to melt the interlayer but not any partof the TENSYLON tapes. The results are shown below.

TABLE 4 Assembly Peel Tensile Tensile Sample temperature strengthmodulus strength configuration (° C.) (N/10 mm) (GPa) (MPa) Standard 154 10 +− 2.7 29.6 +− 3.9 535 +− 55 compaction technique (comparison) WovenPE 152 10.6 +− 1.5  26.8 +− 1.6 483 +− 28 cloth + interlayer Woven PE135 5.9 +− 0.9 14.5 +− 2.7 283 +− 25 cloth + interlayer (comparison)

Compacting at a temperature just above the melting temperature of theinterlayer but below the melting range of the oriented tapes (135° C.)gives modest mechanical properties. The sample made at 152° C. with theinterlayer shows comparable values of tensile modulus, strength and peelstrength, compared with the normal compacted sample made at 154° C.Using the film therefore gives the prospect of lowering the compactiontemperature 2° C., increasing the width of the processing window.

Example Set D

Tests were carried out to investigate the impact of using interleavedlayers of polypropylene (PP) film in combination with the normal layersof woven PP tapes. The PP film this time was the same polymer grade asused for the drawn and woven tapes. The polymer, grade 100GA02, wasobtained from BP Chemicals, Grangemouth, UK.

The film had the following properties:

Mn=78,100

Mw=360,000

Density=910 Kg/m³

It was extruded using a Brabender single screw extruder and a standardfilm die set to a temperature of 260° C. Extrusion screw and wind upspeeds (8 rpm and 4.6 m/min) were chosen such that a film thickness ofapproximately 15 μm was produced.

The next stage in the study was to manufacture a range of samples, withthe film as an interlayer, and without (comparison), to assess theimpact of an interlayer on compacted sheet properties. DSC tests,carried out a heating rate of 10° C./min, showed that the peak meltingpoint of the film was 162° C., while the constrained peak melting pointof the oriented tapes was 194° C. Compacted samples were therefore madeat a temperature of 175° C., high enough to melt the film completely butnot high enough to cause any melting of the oriented phase.

The material used was a fabric woven tape, formed from a slit film, drawratio nominally 10:1, woven in a 6060 style. A single pressure process(4.6 MPa) with a dwell time of 5 minutes was used. Samples were alsocompacted at 180, 187, 189, 191, 193, 195, 197 and 200° C. Cooling ratewas 50° C./min, achieved by passing cold water through the heatingplatens.

In the first set of tests, 4 layer samples were made for measurement ofthe interlayer bond strength, using the peel test. The results are givenin FIG. 5.

It is seen that at all compaction temperatures, the peel strength ishigher when using the interlayer.

The next stage was to measure the stress-strain behaviour of variousmaterials to see if these had been reduced in any way.

The results are shown in FIGS. 6 and 7.

As shown in FIG. 6, within the experimental scatter no significantdifference was seen between the initial tensile modulus of the twogroups of samples. The modulus is seen to be relatively constant between191 and 197° C. for both sets of samples. Thus in this set of tests theintroduction of a thin film of material between the woven layers has nodiscernible detrimental effect on the compacted sample modulus.

For the tensile strength results shown in FIG. 7 there was a clearerdifference seen between the two sets of samples. Here the samples madewith the film showed a higher tensile strength than those compactednormally. This difference is largest at the lower temperatures whenthere is little surface melting of the oriented tapes. However, even inthe ‘optimum’ compaction range, the film samples still show a slightlyhigher tensile strength.

The table below presents a summary of the results from the tensile andpeel strength tests (ASTM protocols as noted above), in respect of peelstrength, tensile modulus, tensile strength and failure strain.

In an attempt to discern the optimum combination of the four parametersmentioned above, and help assess the impact of the interleaved film, thefollowing performance index (PI) was derived. Assuming each propertytested is equally important, this is as followsPT=100×[(E _(T) /E_(max))+(σ_(T)/σ_(max))+(ε_(T)/ε_(max))+(Peel_(T)/Peel_(max))]/4where the subscript T refers to a particular compaction temperature andthe subscript max refers to the maximum value measured for all thesamples. Values of the performance index are also shown in the tablebelow and in FIG. 8. It is seen that the interlayer samples have abetter balance of properties compared to the normal samples whenanalysed in this way, but with the peel strength showing the most markedimprovement.

It will be seen that the PI value of the samples made in accordance withthe invention, employing a film as interlayer, exceeded thecorresponding “no film” value at each given compaction temperature. Thebest performance was achieved when some melting of the woven fabric tookplace, notably at a compaction temperature of around 189-197° C. The PIvalue was higher in the “interlayer” sample.

TABLE 5 Compaction Tensile Tensile Peel Performance temperature modulusstrength Failure strength Index (° C.) E (GPa) σ (MPa) strain ε (N/10mm) (PI) No film‡ 175 2.99 67 5 0.63 38 180 2.31 93 12 1.17 46 187 2.24123 15 1.89 55 189 2.87 148 18 3.7 69 191 3.41 154 18 4.98 76 193 3.43155 15 7.53 77 195 3.4 138 21 7.2 80 197 3.39 137 20 >7.2* 79 200 1.4 2920 >7.2* 49 with film‡ 175 3.09 100 7 5.21 53 180 2.59 155 16 6.23 70187 2.47 145 17 8.66 72 189 3.1 163 18 11 84 191 3.13 168 18 12.3 87 1933.18 173 20 13.7 93 195 3.44 150 19 16.6 94 197 3.49 136 20 >16.6* 94200 1.4 29 20 >16.6* 63 *samples too well bonded to be tested in peeltest ‡comparisonsSEM Images of Polypropylene Peel Fracture Surfaces

The samples compacted at 175, 191 and 193° C. were selected for SEMmicroscopy of their fracture surfaces following peel testing. Thesamples were as follows.

TABLE 6 Sample Compaction temperature (° C.) Details comparison 175 Nofilm comparison 175 1 layer 100GA02 comparison 191 No film invention 1911 layer 100GA02 comparison 193 No film Invention 193 1 layer 100GA02

The measured peel strengths for these samples are as shown in the Tablebelow.

TABLE 7 Compaction temperature (° C.) Without film With film 175 0.63 +−0.12  5.21 +− 0.98 191 4.98 +− 1.6  12.3 +− 4.1 193 7.53 +− 3.52 13.7 +−3.5Peel Fracture Loads (N/10 mm)

The associated SEM micrographs are FIGS. 9-18. Comments on thesemicrographs are as follows.

175° C.—No Film

FIG. 9: This is a low magnification micrograph (×50) showing the sampleedge and fracture surface. The key point is that at this compactiontemperature of 175° C., the tapes and the layers are very poorly bonded.

FIG. 10: This micrograph (×30) shows the peel fracture surface for thesample made at 175° C. without a film. There is very little surfacedamage. As will be seen from the later micrographs, the amount ofsurface damage correlates very well with the peel strength, as beingevidence of the amount of energy needed to separate the surfaces. If thewoven layers are poorly bonded, the failure proceeds between the layerscausing little damage and a low peel load. If the layers are wellbonded, the failure path has to move into the oriented tapes, or thefilm layer, which increases the peel load and the samples then show amuch rougher surface appearance.

175° C.—with Film

FIG. 11: This is a low magnification micrograph (×50) of the sampleedge. It is seen again, that at this temperature the layers and tapesare in general poorly bonded.

FIG. 12: This micrograph (×30) shows that there is considerable surfacedamage associated with the interface where the film was located, whichcorrelates with the measured increase in peel strength. However it isalso seen that the tapes themselves are not well bonded to thoseunderneath, i.e. where there is no film. To Summarise—175° C. Results

Using a film, and processing at a temperature above the film meltingpoint but below the temperature where the oriented tapes melt, gives astructure which is well bonded where the film is present, but poorlybonded elsewhere. It is clear that it would be very difficult for thefilm to penetrate through the woven tape layers.

Processing at a temperature well below the melting temperature of theoriented tapes, and using no film, gives poor bonding throughout thestructure. 191° C.—no Film

FIG. 13: This is a low magnification micrograph (×50) showing the sampleedge and fracture surface. The key point is that at this compactiontemperature of 191° C., where the surfaces of the oriented tapes are nowbeginning to melt, the layers are now much better bonded and thecompacted sheet is more homogeneous. The individual tapes in thecompacted sheet are less apparent than at 175° C. (FIG. 10).

FIG. 14: This micrograph (×30) shows the peel fracture surface for thesample made at 191° C. without a film. As would be expected, there isincreased surface damage compared to the sample made at 175° C. As withmost traditionally compacted samples (i.e. without a film) the surfacedamage is patchy: there are some regions where the damage is pronouncedand others where it is less so. 191° C.—with Film

FIG. 15: This is a low magnification micrograph (×50) of the sampleedge. It is seen that at this temperature the layers are well bonded;the structure is now homogeneous.

FIG. 16: This micrograph (×30) shows that a sample made at 191° C. witha film develops a large amount of surface damage on peeling, reflectingthe higher peel force measured for this sample. The damage is now seento be more even across the sample surface. Perhaps the introduction ofthe film at the interlayer is able to even out any local differences inthe way the two woven layers fit together. To Summarise—191° C. Results

Using a film, and processing at a temperature where the oriented tapesbegin to melt, produces the combination of an overall homogeneousstructure and interlayer regions (the weak point in the structure) whichare very well bonded.

The level of damage (i.e. bonding) is more even over the surface whenusing an interleaved film

The level of damage for the sample made at 175° C. with a film issimilar to that seen for the sample made at 191° C. without a film,reflecting the similarity in the peel load values. 193° C.—without Film

FIG. 17: This shows (×30) a peel fracture surface from a sample made at193° C. without a film. The fracture surface shows a similar amount ofdamage to that on the sample made at 191° C. without the film (FIG. 14)but not as much as that on the sample made at 191° C. with the film. Theamount of surface damage correlates well with the measured peel loads.As with the sample made at 191° C. without the film, the damage seenover the area is patchy. 193° C.—with Film

FIG. 18: This micrograph (×30) which shows regions where there has beencohesive failure within the film and regions of adhesive failure at thefilm/tape interface. This suggests that the failure could be acombination of these two modes. To Summarise—193° C. Results

Using a film, and processing at a temperature where the oriented tapesbegin to melt, produces the combination of an overall homogeneousstructure and interlayer regions which are well bonded.

The level of damage (i.e. bonding) is more even over the surface whenusing an interleaved film. It is proposed that the interleaved film isable to more easily fill any gaps which might be present when the wovenlayers are pressed together.

The level of damage seen on the 193° C. compacted sample fracturesurfaces is higher than that on the corresponding 191° C. surfaces(FIGS. 15, 16) reflecting the associated increase in peel strengths.

Example Set E

In this example set the flexural properties of samples compacted atdifferent temperatures, with and without an interlayer, were tested.

The sample preparation was as described previously. The ASTM testingregimes noted above were used.

FIG. 19 shows the results for both the flexural modulus and flexuralstrength. Below the onset of selective surface melting of the orientedtapes (about 187° C.), the flexural properties of the interleaved filmsamples are better than the conventionally compacted samples. Above thistemperature, the flexural properties of the two sets of samples are verysimilar. Flexural properties peak at a compaction temperature of 195° C.for both sets of samples.

Example Set F

In this set of tests effect of interlayer thickness was studied, usingthe same method and polypropylene material as was used in Example Set D.As with the examples above a film of thickness 10-15 pm was used as aninterlayer, with 0-3 such films being used, multiple films being placedtogether in a stack.

Average values for stress-strain behaviour and peel strength are shownbelow in the following table.

TABLE 8 Compaction Tensile Tensile Peel Temperature modulus strengthstrength (° C.) Interlayer (GPa) σ (MPa) (N/10 mm) 191° C. No film 3.41+− 0.25 154 +− 8 4.98 +− 1.6 1 layer 3.13 +− 0.05 168 +− 8 12.3 +− 4.1 2layers 3.17 +− 0.15 135 +− 9  8.8 +− 1.3 3 layers 3.00 +− 0.36 137 +− 312.5 +− 4.7 193° C. No film 3.43 +− 0.29 155 +− 7  7.53 +− 3.52 1 layer3.18 +− 0.09 173 +− 4 13.7 +− 3.5 2 layers 3.22 +− 0.18 144 +− 5  9.6 +−2.3 3 layers 3.01 +− 0.37 160 +− 9 11.7 +− 4.3

The results indicate that the tensile modulus falls for bothtemperatures as the film thickness is increased; that the tensilestrength peaks for the single layer film thickness and then falls againas the thickness is increased; and that the peel strengths are similarfor all layers of film thickness, and all significantly higher than thecomparative samples without an interlayer.

The results, taken together, suggest that the single layer is theoptimum, giving the maximum increase in peel strength for the minimumloss of tensile modulus, and with retention or slight improvement intensile strength.

Example Set G

In this example set SEM microscopy was used to study peel fracturesurfaces using the same materials and processing as described in ExampleSet B but having multiple interlayers. The processing temperature was193° C., so the figures of Example Set D which provide comparisons areFIG. 17 (no film) and FIG. 18 (one layer of film). FIGS. 20 and 21 areviews of a corresponding peel tested product, but having two and threelayers of the 100GA02 polymer film, respectively. By way of comparison,in the single layer sample of FIG. 18 of Example Set D, one can see thefilm layers F lying on top of the oriented tapes underneath. In FIG. 20a sample made with two layers of film the edge of the sample clearlyshows the film layer F located within the sample, and the film layer onthe peel surface itself. It appears that failure has proceeded at thispoint predominantly through the film layer. From this micrograph we cansee that the damage zone is located within the film layer. FIG. 21 showsa region of the surface showing the thick film layer, now composed ofthree film layers F. The damage zone is now seen to be much thinner thanthe overall film thickness.

Example Set H

This example set examined the importance of the type of film used. Insome of the tests the interlayer was made from the same polymer as wasused to make the oriented tapes (PP 100GA 02 material as describedabove). In other tests two further interlayer films were investigated,namely.

1) A (30 μm thick) polypropylene film of m.p. 163° C., obtained fromICI.

2) A PE film made in-house: this employed the Brabender single screwextruder and the same film die used to make the PP film described above.This used a BOREALIS PE (Film grade FL5580) and the final extruded filmwas between 10 and 15 μm thick.

Compaction experiments were carried out using the same woven PP cloth asdescribed above (10:1 drawn tape, 6060 style, 100GA 02 polymer).Experiments were conducted at two compaction temperatures: 175° C., forcomparison, enough to melt each film but not enough to melt the surfacesof the oriented materials and 193° C. which is in the optimum value fornormal hot compaction

The results axe shown in the table below.

TABLE 9 Film Peel Sample thick- strength thickness ness E (Gpa) σ (MPa)ε (N/10 mm) (mm) μm 175° C. no 2.99 67 5 0.6 0.64 10-12 film‡ matching3.09 100 7 5.2 0.64 30 PP film‡ ICI PP 2.45 86 1.3 0.72 30 film‡ PEfilm‡ 2.51 92 0.7 0.68 10-15 193° C. no 3.43 155 15 7.5 0.47 film‡matching 3.18 173 20 13.7 0.51 10-12 PP film ICI PP 3.08 103 23 8.7 0.5830 film PE film 2.70 113 28 2.3 0.53 10-15 ‡comparisons

The results indicate that the best samples are those made with thematching PP film.

Example Set I

In this example as assessment was made of the application of theinvention to polyester (PET) materials.

Woven PET fabric, and polymer of an identical chemical composition, weresupplied by KOSA, GmbH and Co. KG.

The polymer and fabric details were as follows

TABLE 10 Polymer Type T51-IV ~0.85, Mn ~22,500 Fabric weight 200 g/m²Oriented shape multifilament bundles 1100 decitex Weave style Plainweave 9/9 threads/cm Peak m.p. 250° C.

PET film (.about.15 μm thick) was cast from the polymer using a standardextruder and a film die, A second PET film, of a different chemicalcomposition to the woven cloth, was also used in these tests: this filmwas slightly biaxially oriented.

The work reported looked at the application of the invention to thewoven PET material, both with and without an interleaved film. Sampleswere made using both films.

The table below shows a comparison between the stress-strain and peelstrength behaviour of samples made with and without the film of the samecomposition at 257, 258 and 259/260° C. As is seen all the samples madewith the film showed increased tensile modulus, tensile strength andpeel strength over the samples made without the film, at a giventemperature.

TABLE 11 Compaction Tensile Tensile Peel temperature modulus strengthstrength (° C.) Sample (GPa) (MPa) (N/10 mm) 257 No film‡ 4.51 +− 0.18 88 +− 18 1.2 +− 0.2 Same film 5.69 +− 0.52  178 +− 16 5.1 +− 0.6 258 Nofilm‡ 4.96 +− 0.4  120 +− 5 2.0 +− 0.4 Same film 6.65 +− 0.69 175 +− 55.9 +− 1.4 260/259 No film‡ 6.41 +− 0.77  138 +− 16 7.2 +− 1.2 Same film7.27 +− 0.64 188 +− 8 6.9 +− 0.9 ‡comparisons

As a further experiment samples were also made, using a compactiontemperature of 257° C., using no film, and both PET films, and tested inthe manner described previously. The results are as follows.

TABLE 12 Tensile Tensile Peel modulus strength strength Samples (GPa)(MPa) (N/10 mm) No film 4.51 +− 0.18  88 +− 18  1.2 +− 0.17 Differentfilm 6.85 +− 0.32 158 +− 13 3.9 +− 0.6 Same film 5.69 +− 0.52 178 +− 165.1 +− 0.6

It can be seen that in this experiment the mechanical properties weresignificantly boosted by the presence of either film; and that the filmsgave rise to enhancement of different mechanical properties. Namely thetensile modulus of the sample with the different film is higher thanwith the identical film, although the tensile strength and peel strengthare higher with the identical film.

A significant finding is that these mechanical properties were achievedusing a compaction temperature of 257° C. The optimum temperature forcompacting PET by the prior method (no film) is regarded as 260° C. WithPET the processing window is narrow, which could inhibit thecommercialization of hot compaction processes as applied to PET. Alowering of the compaction temperature to 257° C., yet with achievementof good mechanical properties, suggests a significant practical benefit.

Example Set J

SEM Images of Polyethylene Peel Fracture Surfaces

Peel samples were manufactured as described in Example Set B using wovenTENSYLON 10:1 PE tapes (6060 style). Samples were made with and withoutan interleaved film. In these tests a film of the same grade as theoriented tapes was not available and so the Borealis FL5580 material, asimilar grade, was sourced.

8 samples were studied, having been compacted at 135° C., 148° C., 152°C. and 154° C., with and without an interlayer film, and subjected tothe peel test.

TABLE 13 Compaction temperature (° C.) Without Film With film 135 0.72+− 0.31 5.94 +− 0.92 148 4.23 +− 0.78 9.02 +− 1.18 152 5.56 +− 1.05 10.6+− 1.5  154   10 +− 2.73 13.4 +− 3.3  Peel fracture loads (N/10 mm)

The associated SEM micrographs are FIGS. 22-37 herein. Comments on thesemicrographs are as follows.

FIGS. 22-25: These figures show low magnification micrographs of typicalfracture surfaces from samples made without a film at 135, 148, 152 and154° C. respectively. As the compaction temperature is increased thelevel of surface damage increases. At the lowest temperature, wherethere is no surface melting of the PE tapes, there is no bonding of thetapes.

At 148° C., where the surfaces of the tapes are just beginning to melt,the tapes appear better bonded although the peel surfaces are clear ofdamage.

At 152° C. surface damage has increased, reflecting the increase in themeasured peel load. As with the PP studies, the areas of surface damageare variable when a film is not used.

At 154° C. the damage is further increased.

FIGS. 26-29:

These four micrographs show samples made with a film at 135, 148, 152and 148° C. respectively. All show increased surface damage compared tothe equivalent samples made at the same temperature. Unlike the PPstudies, the film is still visible on some of the fracture surfaces,particularly at 135° C. As the compaction temperature is increased theamount of damage increases. Only at 154° C. is substantial damage seenwithin the oriented tapes (i.e. at the temperature where there issubstantial surface melting of the tapes).

For the other temperatures the failure mode seems to have occurred atthe film/woven cloth surface, i.e. at least partial adhesive failure.The best performance is therefore confirmed as a combination of filmmelting and melting of the tape outer surfaces.

FIG. 30: 135° C.

no film: shows one tape going underneath another at 90° to it, andconfirms no bonding between the tapes at this temperature.

FIG. 31:

135° C. with film: this high magnification micrograph shows surfacedamage and tearing of the interleaved film, but that failure hasoccurred between the film and the woven layer in some instances.

FIG. 32:

148° C. no film: this micrograph shows a junction between tapes andindicates much better bonding between the tapes. However there isminimal surface damage suggesting the surfaces were fairly easilyseparated (i.e. low peel strength).

FIG. 33: 148° C. with film: shows increased surface damage but stilladhesive failure.

FIG. 34: 152° C. no film: increased surface damage on this samplecompared to the lower temperatures made without a film.

FIG. 35: 152° C. with film: shows adhesive failure.

FIG. 36: 154° C. no film: optimum temperature without a film:substantial damage of the oriented tapes produced during peeling.

FIG. 37: 154° C. with film: this sample gave the roughest peeled surfaceseen, which correlates with the highest peel load measured. At thiscompaction temperature the failure appears to be cohesive. The piece offilm on the left shows evidence of material peeled off the adjoiningtape on the other surface.

The invention claimed is:
 1. A polymeric article comprising a ply of atleast three layers, the at least three layers comprising, in sequence, afirst layer composed of strands of a first polymeric material, a secondlayer composed of a second polymeric material, and a third layercomposed of strands of a third polymeric material, wherein the polymericmaterial of each of the three layers has a melting temperature and thesecond polymeric material has a degree of polymeric orientation lessthan that of the strands of the first and third polymeric materials;wherein the first and third layers are fabrics which are woven or knitfrom tapes, fibre yarns, or filament yarns; wherein a portion of each ofthe at least three layers has been melted; wherein each of the at leastthree layers comprises the same chemical class of polymer, the chemicalclass of polymer selected from the group consisting of polyethylene,polypropylene, polyoxymethylene, blends thereof, copolymers thereof, andmixtures thereof; wherein the second layer comprises a lower meltingtemperature than that of the first and third layers; wherein the meltingtemperature of the second layer is at least 5° C. lower than the meltingtemperature of the first and third layers; and wherein one of the atleast three layers was biaxially oriented prior to being melted.
 2. Thepolymeric article of claim 1 wherein the chemical class of polymercomprises polypropylene.
 3. The polymeric article of claim 1 wherein thechemical class of polymer comprises a blend of polypropylene andpolyethylene.
 4. The polymeric article of claim 1 wherein the secondlayer has a thickness less than that of either the first layer or thethird layer.
 5. The polymeric article of claim 1 wherein each of the atleast three layers has a substantially identical thickness.
 6. Thepolymeric article of claim 1 wherein the second layer has a thicknessgreater than that of either the first layer or the third layer.
 7. Thepolymeric article of claim 1 wherein at least one of the at least threelayers comprises multiple layers.
 8. A polymeric article comprising aply of at least three layers, the at least three layers comprising, insequence, a first layer composed of strands of a first polymericmaterial, a second layer composed of a second polymeric material, and athird layer composed of strands of a third polymeric material, whereinthe polymeric material of each of the at least three layers has amelting temperature and the degree of polymeric orientation of thesecond polymeric material is less than that of the strands of the firstand third polymeric materials; wherein the first and third layers arefabrics which are woven or knit from tapes, fibre yarns, or filamentyarns; wherein a portion of each of the at least three layers has beenmelted; wherein each of the at least three layers comprises the samechemical class of polymer, the chemical class of polymer selected fromthe group consisting of polyethylene, polypropylene, polyoxymethylene,blends thereof, copolymers thereof, and mixtures thereof; wherein thesecond layer comprises a lower melting temperature than that of thefirst and third layers and is melted to a greater degree than the firstand third layers; wherein the melting temperature of the second layer isat least 5° C. lower than the melting temperature of the first and thirdlayers; and wherein the second layer is not oriented.
 9. The polymericarticle of claim 8 wherein the chemical class of polymer comprisespolypropylene.
 10. The polymeric article of claim 8 wherein the chemicalclass of polymer comprises polypropylene and polyethylene.
 11. Thepolymeric article of claim 8 wherein the second layer has a thicknessless than that of either the first layer or third layer.
 12. Thepolymeric article of claim 8 wherein each of the at least three layershas a substantially identical thickness.
 13. The polymeric article ofclaim 8 wherein the second layer has a thickness greater than that ofeither the first or third layer.
 14. The polymeric article of claim 8wherein at least one of the at least three layers comprises multiplelayers.